Reactor fouling in graft copolymerization of propylene polymer materials with styrenic and other vinyl monomers occurs in both semi-batch stirred tank reactors and in gas mixed loop reactors. The polymer deposits that are formed on the reactor walls and gas circulation loops usually grow rapidly after an initial deposit. Severe reactor fouling would affect the product quality, productivity, and operability of a commercial plant.
Since the main reaction is a free radical polymerization using a peroxide as an initiator, any attempt to reduce reactor fouling by using a monomer-soluble free radical scavenger will potentially interfere with the graft polymerization process. Therefore such soluble scavengers are preferably not used in this process.
It is known that polymerization inhibitors such as nitric oxide can be used to prevent polymerization of vinyl aromatic compounds during distillation, for example, as disclosed in U.S. Pat. No. 3,964,979. U.S. Pat. No. 4,070,419 discloses adding gaseous NO, e.g., during purification of styrene by distillation, and then subjecting the monomer to polymerization conditions, whereby the styrene polymerized at an accelerated rate. Hungarian Patent 77-MA2891 discloses grafting irradiated polypropylene with styrene in the presence of a nitroxyl polymerization inhibitor to reduce side reactions, i.e., no polystyrene by-product was formed. U.S. Pat. No. 5,283,287 discloses a process for making a thermoplastic resin composition having excellent HCFC resistance which includes graft polymerizing a monomer mixture of a vinyl cyanide, a vinyl aromatic compound, and an unsaturated carboxylic acid or ester thereof in the presence of a rubber latex and a polymerization inhibitor such as nitric oxide to control the sequence of polyacrylonitrile units. However, none of these references discloses the use of a continuous feed of nitric oxide to reduce reactor fouling during the production of polypropylene graft copolymers.